Power distribution in today's world generally consists of power mains transmitting alternating current power signals. Computers, LED lights, and other electronic devices commonly use power conversion circuits to transform the electric current from a power main to a voltage signal usable by the device. The power conversion circuitry provided takes up space on the device, and also wastes power in the conversion.
Manufacturers of electronic devices quest for more efficient and higher density power supplies. Increasing the density of power conversion circuits is a firmly established trend in low to medium voltage DC-to-DC power supplies. However, the AC-to-DC market has been largely immune to the trend for three primary reasons. First, electromagnetic interference (EMI) standards are less stringent at switching frequencies under 150 kHz. Switching frequencies have typically been limited so that the fundamental frequency of EMI emissions is below 150 kHz. Second, passive devices, and in particular transformer core materials, have not kept pace with the advancements in semiconductor devices. Third, high voltage switches, e.g., greater than 650 V, had poor Figure of Merit (FoM) until recently. The first obstacle, related to EMI emissions of switch-mode power supplies, is easily overcome with zero voltage switching (ZVS), as is well known. With advances being made in high voltage switches, including new and exciting wide-bandgap semiconductor devices, new topologies of switch-mode power supplies are in play. One such topology is the active clamp flyback (ACF) power converter.
ACF converters have appeared in literature since the mid-1990s. However, ACF converters have not been widely used, and have primarily been seen in low volume designs. The ACF topology is a fixed switching frequency topology that utilizes the energy stored in the parasitics of the circuit to achieve ZVS, rather than dissipating the energy into a snubber circuit. The waveforms resulting from ACF operation show reduced spikes, which improves EMI relative to conventional switch mode power supplies. However, ACF converters utilize two MOSFETs and require an additional half-bridge driver. The additional parts of an ACF converter increased costs and resulted in low usage in cost sensitive and high volume markets such as laptop power adapters and light emitting diode (LED) lighting.
Recent implementations of the classic flyback converter have approached the topology's limits of efficiency and power density. Quasi-resonant flyback is a variant of the classic flyback topology that gets quasi-ZVS and shows potential for pushing power density higher. However, quasi-resonant flyback includes a frequency that varies with load current, and doesn't solve the problem of dissipating leakage energy. On the other hand, the ACF topology, with a fixed frequency operation while achieving ZVS, is very attractive for further development. The ACF topology solves the efficiency puzzle while lowering EMI emissions.